The present invention concerns a Bragg reflection optical device for reflecting at least one predetermined wavelength band present in an incident light, the device including at least one liquid crystal film of the cholesteric type. The invention also concerns methods for manufacturing such optical devices.
It will be recalled that a cholesteric type liquid crystal only reflects, by Bragg reflection, light which has circular polarisation having the same rotational direction as that of the liquid crystal. It will be noted that, in the following description, the reflection coefficient is equal to 1 when a circular polarisation light is completely reflected.
The characteristic of certain cholesteric liquid crystals is having a helical periodic structure having a pitch which can be adjusted. This helical structure causes Bragg reflections whose reflection band, i.e. the wavelength range which it can reflect, can easily be modified by selecting other values for the helical pitch and/or the liquid crystal birefringence.
From such crystals, one can make optical devices, in particular for display, by introducing, between two plates or substrates, several cholesteric liquid crystals having one more pitches adjusted to reflect respectively a wavelength corresponding to a determined colour.
One problem which is commonly encountered when such devices are made lies in the fact that the reflected colour, in particular the colour red, has a dull or faded appearance.
In order to explain this phenomenon, reference will be made hereinafter to FIG. 1, which shows a curve 1 illustrating the reflection spectrum of an optical device with cholesteric liquid crystals adjusted to reflect the colour red. It is to be noted that reflection of wavelengths xcex corresponding to the colour red by such a liquid crystal is imperfect. Indeed, curve 1 can be broken down into a main band A, corresponding to reflection of the colour red, and into two lateral bands B and C on either side of main band A. The effect of the presence of lateral bands B and C is that the colour red reflected by the optical device is not pure, i.e. it is not sufficiently saturated, nor sufficiently brilliant.
It will be recalled that saturation is linked to the limitation of the wavelength spectrum of the colour red, and that brilliance is linked to the whether the reflection coefficient is close to 1 or not.
Moreover, the characteristic of the human eye will accentuate the undesirable effect of these lateral bands on the purity of the red colour which it sees. FIG. 1 shows a curve 2 illustrating the response of the human eye as a function of the wavelength xcex of the light which the eye receives, i.e. for all the colours of the visible spectrum (this curve also being called the photopic curve). It will be noted that the human eye is most sensitive in day vision (photopic vision) to the wavelengths xcex closes to wavelength 555 nm, which corresponds to the peak of curve 2.
FIG. 2 shows a curve 3 illustrating the eye""s perception of the colour red reflected onto the optical device having the feature shown in curve 1. In other words, FIG. 2 shows the reflection spectrum of the colour red multiplied by the human eye""s response, as a function of wavelength xcex. It will be noted in FIG. 2 that the effects of lateral band C of the low wavelengths of the colour red are amplified by the human eye, which adversely affects the colour red and gives it a dull appearance; it then becomes orange-red.
It has been observed that similar phenomena occur with the colour blue. However, the effects of the lateral bands are more amplified for the colour red than for the colour blue, because of the photopic curve of the human eye.
In order to overcome this problem of the purity of colour emitted by an optical device of the aforementioned type, there exist several types of solution in the prior art.
A first solution to this purity problem is described in the work entitled xe2x80x9cLiquid Crystal in Complex Geometriesxe2x80x9d by Taylor and Francis, published in 1996, page 257, and consists in doping the liquid crystal with a dye which is intended to absorb the undesired parts of the reflection spectrum.
One drawback of the first solution is that the optical effect obtained is not optimum. Indeed, it is possible for the light reflected by the liquid crystal not to have met molecules of dye or to only have been modified by a few dye molecules, so that the colour is unsaturated, or, in other words, is not pure.
Another drawback of this solution is that it requires the mixture of the liquid crystal and the dye to be physically separated from other liquid crystals which reflect respectively green and blue, in order to avoid diffusion of the dye molecules in the neighbouring liquid crystals of different colours. This has the effect of increasing the complexity of the device.
Another drawback lies in the fact that the dye has residual absorption for the wavelengths of the main band, which has the effect of reducing brilliance.
Another drawback lies in the fact that this solution involves absorption of the transmitted light, which means that a stack of several liquid crystal cells cannot be used to combine optical effects, for example different colours.
Another drawback lies in the poor chemical stability of the molecules forming the dye, in particular in the presence of ultraviolet rays (UV), which reduces the reliability and lifetime of the display device.
A second solution to the aforementioned purity problem is described in European Patent Application No. EP 0 872 759, in the case of a liquid crystal display device (LCD). This solution consists in providing the LCD device with a filter able to absorb the visible wavelengths different to that corresponding to the colour that the crystal has to reflect. This filter eliminates the effect of the lateral bands described above from the spectrum reflected by the liquid crystal, so as to make the reflected colour more pure.
This second solution also has various drawbacks. It requires the complex arrangement of the absorbent filter, which goes against the usual industrial concerns as to cost, compactness and rationality. Moreover, it requires the arrangement of equalising layers to allow a constant thickness of the liquid crystals to be assured over the entire surface of the cell, which increases the complexity of such a device.
This solution also has the drawback of involving absorption of the transmitted light, which means that a stack of several liquid crystals cannot be used to combine their optical effects.
An object of the present invention is thus to provide an optical device which overcomes the aforementioned drawbacks, in particular an optical device able to reflect or transmit with optimum purity a predetermined colour having a wavelength comprised within the visible range, for example the colour red, or outside such range, for example infrared rays.
Another object of the present invention is to provide a device able to reflect or transmit a predetermined colour with optimum saturation.
Another object of the present invention is to provide a device able to reflect or transmit a predetermined colour the brilliance of which is optimum.
Another object of the present invention is to provide an optical device allowing a plurality of colours to be reflected to transmitted.
Another object of the present invention is to provide an optical device answering the usual concerns in the industry as to cost, compactness and rationality.
According to the invention, an optical device of the type indicated in the preamble is provided, characterised in that the liquid crystal film has, in at least part of its thickness, a birefringence gradient as a function of the depth in said film.
One advantage of the birefringence gradient liquid crystal of such an optical device is that it can limit the reflection spectrum to a wavelength band having very clear limits with neighbouring wavelengths, and it makes the reflection coefficient close to 1. As a result, the band reflected by the optical device is both more brilliant and more saturated, i.e. more pure. Likewise, the non-reflected light can be transmitted through the optical device with great purity.
The birefringence gradient may be negative or positive, the birefringence being respectively decreasing or increasing as a function of the depth counted from the face of the film receiving the incident light. In other words, the effect of removing the lateral bands is obtained in both directions for the light passing through the liquid crystal film.
The birefringence gradient may be constant or variable as a function of the depth counted from the face of the film receiving the incident light.
In a particular embodiment, the film is formed by a plurality of polymerised layers of a cholesteric type liquid crystal able to reflect said predetermined wavelength band, these layers having constant birefringence coefficients, which differ from one layer to another, and ordered gradually so as to form said birefringence gradient.
Generally, the extraordinary refractive index ne may or may not vary linearly as a function of depth z, while the ordinary refractive index no may be constant or variable.
In another embodiment, the optical device includes a cell containing said liquid crystal film, said cell including first and second substrates and a sealing frame which delimit a cavity containing said film. This device may further includes two groups of electrodes arranged respectively on either side of the film and a control circuit connected to said electrodes arranged to provide them selectively with control voltages, so as to cause the liquid crystal to switch from a first state, in which it reflects the light of said band, into a second state in which it is transparent to said light, or vice versa.
In the two aforementioned embodiments, said device can include a stack of a plurality of said liquid crystal films, each of said films being arranged to reflect the light of a different wavelength band.
The present invention also concerns a method for manufacturing an optical device formed by a plurality of polymerised layers as indicated above. This method includes the steps of:
providing a substrate;
depositing on the top surface of said substrate a first layer of polymerisable cholesteric liquid crystal having a first birefringence coefficient and reflecting a predetermined wavelength band, and polymerising said first layer;
depositing and polymerising, in succession on the preceding layer, superposed layers of polymerisable cholesteric liquid crystal reflecting said predetermined wavelength band and having respective birefringence coefficients which vary gradually with respect to that of the preceding layer, so as to form said film by a stack of layers together having a birefringence gradient in the thickness of the film.
The present invention also concerns a method for manufacturing an optical device including a cell as indicated above. This method includes steps of:
manufacturing a liquid crystal cell containing a film formed of a mixture comprising: an agent filtering ultraviolet radiation, a first cholesteric liquid crystal having a first birefringence coefficient and reflecting a predetermined wavelength band, and a second cholesteric liquid crystal having a second birefringence coefficient different from said first birefringence coefficient and reflecting said predetermined wavelength band;
generating polymerisation of the second liquid crystal in a top portion of the film by irradiating said mixture by ultraviolet radiation from a top face of the cell, so that the second liquid crystal is mostly fixed in said top portion by polymerisation and so that its concentration decreases with the depth in the film.
Such a partial segregation method via polymerisation varying with the depth is known in particular from European Patent No. EP 606 940 and U.S. Pat. No. 5,691,789, but in manufacturing methods using a mixture of two liquid crystals having different pitches, and not different birefringence. Indeed, in these methods the object is the manufacture of cholesteric polarisers which have a cholesteric helix pitch gradient in order to offer a much greater band width than that of the prior art. This intrinsically goes against the object of the present invention, which aims to limit precisely the band width of the optical device. However, the polymerisation methods which decrease progressively with the depth which are mentioned in these prior publications can be used to implement the invention disclosed here.